Polymer films have found application in myriad uses, e.g., from packaging to multilayer optical films. The manufacture of such films entails formation of single or multiply layered films and handling of such films by passing through roller apparatuses for application of additional layers, exposure to selected treatments to impart desired performance and properties, conversion to select size and configuration, storage, and transport. In typical operations, the polymer film is wound around rollers as it passes through various equipment and is often wound upon itself into roll form.
Important uses of polymeric films include so-called optical films that are used in a wide variety of applications, e.g., mirrors, polarizers, etc., that rely upon uniform film properties. U.S. Pat. No. 5,882,774 (Jonza et al.), U.S. Pat. No. 6,080,467 (Weber et al.), U.S. Pat. No. 6,268,961 (Nevitt et al.), U.S. Pat. No. 6,368,699 (Gilbert et al.), U.S. Pat. No. 6,673,425 (Hebrink et al.), U.S. Pat. No. 6,827,886 (Neavin et al.), and U.S. Pat. No. 8,182,924 (Hebrink et al.) disclose illustrative examples of such films. Illustrative examples of brightness enhancement films and optical assemblies comprising such films are disclosed in U.S. Pat. No. 5,161,041 (Abileah), U.S. Pat. No. 5,771,328 (Wortman et al.), U.S. Pat. No. 5,828,488 (Ouderkirk et al.), U.S. Pat. No. 5,919,551 (Cobb et al.), U.S. Pat. No. 6,277,471 (Tang), U.S. Pat. No. 6,280,063 (Fong), U.S. Pat. No. 6,354,709 (Campbell, et al), U.S. Pat. No. 6,759,113 (Tang), U.S. Pat. No. 7,269,327 (Tang), U.S. Pat. No. 7,269,328 (Tang), and US Patent Appln. Publn. No. 2002/0057564 (Campbell et al.).
Such films exhibit desirable optical properties as selected based on the precise structure of features, e.g., thickness and arrangement of layers with differing refractive indices, etc. The surfaces of films may be marred as they are handled and used, degrading their suitability for the desired ultimate purpose. Accordingly, it is known in the art to provide layers on the outside of such films, sometimes referred to as “premask” or “skin” layers, to protect the films and facilitate handling and use.
The term “skin” or “skin layer” is often used to refer to a layer of a 3- or more-layered film which is at an outer surface of the film. In the case of multilayer optical films, the skin layer or layers (on one or both sides of the film) can be a protective boundary layer or layers introduced during coextrusion and/or layer multiplying that aid in extrusion of the optical layers, or the skin layer or layers can be an additional layer or layers which can be added, for example, in the coextrusion feedblock, or in a subsequent feedblock, or in the die, or can be added after coextrusion of the multilayer film by extrusion coating or other techniques. Skin layers can serve a variety of purposes, such as protection of the interior layers, for example, against scratching or abrasion, stiffening or strengthening or providing dimensional stability to the entire film, hosting additives such as ultraviolet light stabilizers, reducing surface reflection, providing an appropriately receptive surface for subsequent coatings, or others.
The term “premask” is often used to refer to a surface layer, either produced with the rest of the film or added later by such means as lamination, extrusion coating, solvent coating, or other means, intended to protect the film temporarily against damage during handling, processing, converting and the like, and is further intended to be removed before final use of the film or of portions cut from the film. A so-called “strippable skin” can serve as a premask. It will be understood that a film can also be prepared with a skin layer, say, by coextrusion, and have a premask applied thereon, say, by lamination, whereupon the skin layer is no longer, temporarily, the film construction's external-most layer, but is intended at a later time once again to serve as the external layer, after removal of the premask.
A challenge with polymer films is that when contacted to other films, e.g., such as when wound into roll form or arranged into a stack of sheets, the films may tend to adhere strongly if they are too close to having perfectly smooth surfaces. This is sometimes referred to as “blocking”, making a roll difficult to unwind or a sheet difficult to remove from the stack. Alternatively, if a film surface is made too slippery, film on a roll may tend to slip upon itself, sideways, resulting in what is often referred to as roll “telescoping”. Good roll formation with acceptable unwindability requires that the film surface strike a balance.
U.S. Pat. Nos. 5,328,755 and 5,332,617 (both Mills et al.) disclose formation of particle-filled layers on polymeric films to increase the surface roughness thereof to improve the handling properties thereof.
In some cases, portions of a sheet are roughened, e.g., by imparting knurled portions on the edges thereof, to make the sheets easier to handle and to impart a tendency to “stand off” from articles into which they are contacted, e.g., other sheets in a stack or other portions of the same sheet when wound into roll form. The use of such measures frequently degrade the subject portions of the sheet, thereby reducing yields and incurring additional costs.
The need exists for improved ways to improve the handling properties of polymer films and sheets. In particular, there is a need for a way of improving the handling properties of polymer films that allows a particle-filled layer to be coated onto a film at some point in the film-making process while minimizing the amount of binder polymer needed in the coating.